docs: def predict_y(self, xq, **kwargs):
          """Provide an prediction of xq in the output space
  
          @param xq  an array of float of length dim_x
          @return    predicted y as np.array of float
          """
          _, indexes = self.dataset.nn_x(xq, k = 1)
          return self.dataset.get_y(indexes[0])